Organic light-emitting diodes (OLEDs) containing red-light-emitting dioxolane-substituted pentacene derivatives are fabricated and characterized. The OLEDs feature guest–host emitting layers consisting of either 6,14-bis(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (TP-5) or 2,2,10,10-tetraethyl-6,14-bis(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (EtTP-5) dispersed in tris(quinolin-8-olato) aluminum(III) (Alq3). High external electroluminescence (EL) quantum efficiency (ηEL = 3.3 %), not far from the theoretical limit, is observed for an OLED device based on a dilute EtTP-5:Alq3 emitting layer (0.25 mol % EtTP-5). The proposed EL mechanism is a combination of Förster energy transfer and direct electron–hole recombination on the guest pentacene molecules, as inferred by changes in the EL versus photoluminescence spectra and the positions of the highest occupied molecular orbital and lowest unoccupied molecular orbital gap of the guest within that of the host (estimated via cyclic voltammetry). Further evidence of charge trapping is provided by increased operational voltages at increased guest-molecule concentration.